Field of the Invention
The present invention relates to a light scanning apparatus including a rotary polygon mirror.
Description of the Related Art
Hitherto, an electrophotographic image forming apparatus such as a digital copying machine, a laser beam printer, or a facsimile apparatus includes a light scanning apparatus. The light scanning apparatus scans a surface of a photosensitive drum (hereinafter referred to as “photosensitive member”) with a laser beam (hereinafter referred to as “light beam”) modulated according to the image information, to thereby form an electrostatic latent image thereon. The electrostatic latent image is developed by a developing device into a toner image with developer (hereinafter referred to as “toner”). The toner image on the photosensitive member is transferred onto a recording medium, such as a paper sheet, by a transfer device. A fixing device fixes the toner image transferred onto the recording medium, to thereby form an image on the recording medium.
The light scanning apparatus includes a light source configured to emit a light beam and a rotary polygon mirror configured to deflect the light beam emitted from the light source so that the light beam scans a photosensitive member. The light scanning apparatus generates a synchronizing signal (hereinafter referred to as “BD signal”) in a main-scanning direction in order to maintain the same image writing start position (scan starting position) on the photosensitive member with respect to each scan. When the light beam enters a beam detector (hereinafter referred to as “BD”), the BD signal is output from the BD.
The rotary polygon mirror has a plurality of reflection surfaces, and is manufactured (processed) so that the respective reflection surfaces have the same optical characteristic. However, due to a manufacturing error, the optical characteristic of any one of the reflection surfaces may differ from the optical characteristic of another reflection surface (in reflectance or in angle of the reflection surface with respect to a rotation axis). In view of this, the reflection surface of the rotary polygon mirror is identified to correct the optical characteristic of the reflection surface (Japanese Patent Application Laid-Open No. 2001-260413). In order to identify the reflection surface of the rotary polygon mirror, some apparatus use the BD signal (Japanese Patent Application Laid-Open No. 2013-109113). In general, the rotary polygon mirror is formed in a regular polygonal shape. However, in Japanese Patent Application Laid-Open No. 2013-109113, in order to facilitate the identification of the reflection surface with use of the BD signal, the rotary polygon mirror is formed in a non-regular polygonal shape so as to provide a difference between a first interior angle and a second interior angle that are adjacent to each other.
However, when the difference between the interior angles of the rotary polygon mirror is small, the manufacturing error may cause repetitive occurrence of the same pattern in a BD signal period per revolution of the rotary polygon mirror. In such a case, it is difficult to identify the reflection surface. On the other hand, when the difference between the interior angles of the rotary polygon mirror is large, identification of the reflection surface is facilitated, but image defects may be caused.